1. Field of the Invention
This invention relates to a circuit board with high strength and high reliability using a silicon nitride (Si.sub.3 N.sub.4) sintered body as an insulating layer, a semiconductor device using the same and a process for preparing the same, particularly to a circuit board with high strength and high reliability having single layer wiring or multi-layer wiring obtained by integrally sintering an insulating layer(s) and a conductor layer(s).
2. Prior Art
Accompanying with miniaturization of electronic devices, it is an important problem of how to effectively dissipate exothermic heat from semiconductor elements mounted to a circuit board. Also, in the case of mounting semiconductor elements for electric power, heat dissipation is an important problem.
As an insulating material for a circuit board, Al.sub.2 O.sub.3 ceramics has been widely used. Here, Al.sub.2 O.sub.3 has a low thermal conductivity which is at most 20 W/mK so that there is a problem in heat dissipation. Also, in Japanese Laid-Open Patent Publication (Kokai) No. 178688/1985, application of AlN ceramics having excellent various electric characteristics such as electric insulating characteristics required as an insulator and excellent in thermal conductivity to a circuit board has been investigated.
However, AlN or Al.sub.2 O.sub.3 is spoiled by thermal stress caused by exothermic heat from semiconductor elements and in AlN ceramics, there is a problem that the strength thereof is low as 300 MPa or so with the four points bending strength and when thermal stress is focused, crack will occur. This problem is not limited only to AlN, and in Al.sub.2 O.sub.3, the same cracking phenomenon due to the reason of low strength of a sintered body can be observed. Also, AlN ceramics has poor chemical resistance such as water resistance, acid resistance and alkali resistance. Further, when external terminals such as pins, leads or balls are connected with the ceramics by a metal such as a silver wax or solder, it has a problem that the connecting portions are likely broken due to a stress caused by the difference in a thermal expansion coefficient between the ceramics and the metal.
A ceramics substrate in which the above problems are substantially overcome has been disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 212441/1992. This ceramics substrate is mainly composed of Si.sub.3 N.sub.4, and has high heat dissipation property than an alumina substrate and is excellent in environmental resistance, mechanical strength and electric characteristics.
On the other hand, when miniaturization and high density assembly of an electronic device are considered, high density assembly is also required for wiring of a circuit board and multi-layered constitution is essential technique. However, the conventional multi-layered technique is to integrally sinter Al.sub.2 O.sub.3 ceramic insulator layers or AlN ceramic insulator layers with conductor layers, and a technique for making multi-layer of Si.sub.3 N.sub.4 ceramic layers and conductor layers has not yet been established. Also, in the conventional technique for making multi-layer, a conductor layer suitable for a Si.sub.3 N.sub.4 ceramic layer has not yet been developed. Thus, in a circuit board or a semiconductor device produced by the multi-layered technique, there are problems of a highly resistant conductor layer, or low dimensional precision, occurrence of warpage, burn out or peeling of a conductor circuit due to mismatching in shrinkage factors between an insulator layer and the conductor layer.